No guilt needed.
This is a prototype NASA owned one of 56 watches. Most of us would shit our pants just to wear it on our wrist for a day. If someone doesn't understand or appreciate the significance of this watch then that's on them. Is it someone elses responsibility to teach them to respect the history, their history? Hell no. In this age where everything is available on the internet, you must be too lazy or uncaring to do the minimal research, that's on you. What's more, if you only care about how much money you can make ( or didn't make because you sold it too cheap), no mercy. You didn't deserve the watch. People will pay 250k to 500k for this watch, maybe more. Some might only care about the money. I wouldn't know because I don't have that to spend. But they at least hold the watch in some great value, enough to drop real cash. As a nerdy fan of space and speedmasters, it almost feels insulting to sell something like this cheap. Especially if it was family.
Buy him a beer. Maybe he'll appreciate his Grandfather more now.